May 31, 2013

Amazon Tree Seeds Become Smaller, Weaker Because Of Bird Decline

Over the last century, the disappearance of large, fruit-eating birds from the tropical forests of Brazil has caused the region´s forest palms to produce smaller, less successful seeds, according to an international team of researchers. The findings, published in Science, provide evidence that human activity can trigger fast-paced evolutionary changes in natural populations.

Mauro Galetti, biological sciences professor from the Universidade Estadual Paulista (UNESP) in SÃ£o Paulo, worked with colleagues to use patches of rainforest that had been fragmented by coffee and sugar cane development during the 1800s to set up their natural experiment. The team collected more than 9,000 seeds from 22 different Euterpe eduli palm populations. They then used a combination of statistics, genetics and evolutionary models to determine that the main reason for the observed decrease in the size of the palm´s seeds was due to the absence of the large, seed-dispersing birds.

"Unfortunately, the effect we document in our work is probably not an isolated case," said Galetti. "The pervasive, fast-paced extirpation of large vertebrates in their natural habitats is very likely causing unprecedented changes in the evolutionary trajectories of many tropical species."

Previous research has estimated that human activity, such as deforestation, drives species to extinction approximately 100 times faster than the natural evolutionary processes. Very few studies, however, have been successful at documenting such rapid evolutionary changes in ecosystems that have been modified by human activity.

The current team found that the palms produced seeds that were significantly smaller in patches of forest that had been fragmented by coffee and sugar cane plantations. These palms were no longer capable of supporting large-gaped birds, or those whose beaks are more than 0.5 inches wide, such as toucans or large cotingas. In patches of forest that remain undisturbed by human activity, however, large-gaped birds still make their homes and the palms continue to produce large seeds that are successfully dispersed by the birds.

"Small seeds are more vulnerable to desiccation and cannot withstand projected climate change," explained Galetti. Large seeds, however, are unable to be swallowed and dispersed by small-gaped birds, such as thrushes, that populate the fragmented patches of forest. This impaired dispersal causes palm regeneration to become less successful in the area, with less-vigorous seedlings germinating from smaller seeds.

The influence of a wide range of environmental factors, such as climate, soil fertility and forest cover, were considered by the team, but none could account for the change in palm seed size over the years in the fragmented forests. Genetic analysis revealed that the shrinkage of seeds among forest palms in the region could have taken place within 100 years of an initial disturbance.

The conversion of tropical forest to agricultural land began in the 1800s, displacing many large bird populations in the region, fitting the timescale established by the genetic analysis. This conversion also triggered a rapid evolution of forest palms that resulted in smaller, less successful seeds.

Climate models have predicted long periods of drought and increasingly warmer climates for South America, which could be particularly harmful to tropical tree populations that depend on animals to disperse their seeds. Approximately 80 percent of the entire Atlantic rainforest biome remains in small fragments. The successful restoration of these habitats critically depends on the preservation of mutualistic interactions between animals and plants.

"Habitat loss and species extinction is causing drastic changes in the composition and structure of ecosystems, because critical ecological interactions are being lost," said Galetti. "This involves the loss of key ecosystem functions that can determine evolutionary changes much faster than we anticipated. Our work highlights the importance of identifying these key functions to quickly diagnose the functional collapse of ecosystems."